Continuous tube forming and coating

ABSTRACT

An improved continuous tube forming and coating process and apparatus, wherein the tube is preferably formed and welded with the seam located at the bottom of the tube. One surface of the strip is coated with a paint or other protective coating and the open seam tube is formed with the painted surface on the inside of the tube. An improved coating applicator and impeder is disclosed which coats the inner surface of the seam following welding preferably in an inert atmosphere. The improved impeder includes an outer annular chamber adjacent the casing which includes one or more ferrite rods and circulating coolant and an inner axial paint applicator tube having a free end which extends through the impeder casing having a nozzle which applies a protective coating over the seam, preferably located in the lower portion of the tube. In one preferred embodiment of the impeder, an inert gas passage surrounds the paint applicator tube. An improved galvanizing apparatus is also disclosed having a sealed galvanizing tank which receives the seamed tube below the level of the molten zinc and which is enclosed within a sealed housing and wherein the inert gas pressure in the galvanizing tank is greater than the inert gas pressure in the housing. The galvanizing apparatus includes improved inert gas seals which may be also used to shape the molten zinc on the tube leaving the housing.

This is a divisional of application Ser. No. 08/083,099 filed on Jun.24, 1993, now U.S. Pat. No. 5,651,819.

The present invention relates to an improved process and apparatus forcontinuously forming a seamed metal tube. More particularly, the presentinvention relates to improvements in forming a seamed metal tube havinga coated inner surface, an improved external galvanizing tank, andimprovements in induction tube welding impeders.

BACKGROUND OF THE INVENTION

Methods of continuously or in-line forming a seamed metal tube from acontinuous strip or skelp are well known. After cleaning and edgeconditioning of the strip, if required, the skelp is rolled to form anopen seam tube having nearly abutting edges at the top of the tube. Theedges are then welded together by one of several methods which generallyinclude heating the edges and either forging the edges together withsqueeze rolls and/or continuous flux welding. The edges of the tube maybe heated for example by resistance welding, an electric arc or by highfrequency induction welding. High frequency welding is a form ofelectric resistance welding, wherein the open seam tube is receivedthrough an electric coil which creates a strong magnetic field, which inturn induces current to flow around the tube. An impeder is generallylocated within the tube which forces the current flow down the nearlyabutting edges of the open seam tube, heating the tube edges to a hotforging temperature. The tube edges are then forged by squeeze rollswhich drive the molten edges together to form an integral seam.

In-line galvanizing and coating or painting processes are also wellknown. The strip or skelp may be galvanized or painted on one or bothsurfaces prior to forming and welding or the welded seamed tube may begalvanized by immersion in a molten zinc bath.

Where the strip is coated with a protective coating prior to seamwelding, the coating will burn off or melt in the seam zone because thewelding operation involves the melting of the tube material, which isgenerally steel. Thus, the welding temperature may be 2,300° F. orgreater. Where the strip has been painted, the paint will burn off inthe weld zone. Where the strip is coated with a metal, such as zinc oraluminum, the metal will melt and flow downwardly away from the seam,which is located at the top of the tube. A zinc coating solution hasalso been used to paint the exterior surface of the seam. However, sucha coating is primarily cosmetic and has poor adhesion. The failure ofpresent processes to fully coat and thus protect the tube seam isevident by the fact that the weld area is generally the first to fail inaccelerated corrosion tests. Thus, there has been a long-felt need toprovide an improved coating process, particularly on the seam.

Further, the art has long recognized the advantages of galvanizingferrous tubing in an inert atmosphere, such as nitrogen. Maintaining areducing or nonoxidizing atmosphere within the galvanizing tank improvesthe resultant zinc coating, reduces oxidization or rust and waste,including slag. However, the prior art has failed to develop acommercially acceptable galvanizing apparatus which can be serviced andwhich maintains an inert or nonoxidizing atmosphere, particularly duringservice.

The continuous tube forming and coating process and apparatus of thisinvention solves the above-identified problems and produces a superiorcoated tube. The process of this invention assures a fully coated innerseam and an improved galvanized exterior tube surface.

SUMMARY OF THE INVENTION

As set forth above, the present invention relates to an improvedcontinuous tube forming and coating process and apparatus. The disclosedprocess includes an improved continuous method of forming a seamed metaltube having a coated inner surface from a continuous moving metal stripor skelp. The improved process includes applying a coating to at leastone surface of the strip, then continuously rolling and forming thestrip into a tube-shaped strip or open seam tube having the coatedsurface located on the inner surface of the tube and the strip lateraledges located in a lower portion of the tube. The process then includesheating and continuously integrally bonding or welding the edges of thetube-shaped strip together to form an integrally seamed tube. Theprocess then includes applying a coating to at least an inner surface ofthe seam by locating a coating applicator between the spaced edges ofthe tube-shaped strip having a coating application nozzle locateddownstream of the weld which projects downwardly toward the tube seamand sprays the coating downwardly onto the seam, coating the seam. Thus,the "upside down" orientation of the lateral edges of the tube-shapedstrip in a lower portion of the open seam tube results in a significantadvantage in the final paint application because excess coating materialwill flow downwardly over the seam and the liquid coating will saturatethe roughened seam, forming a good adhering coating, particularly ascompared with the present processes, wherein the seam is located at thetop of the tube.

In the most preferred process of this invention, the lateral edges ofthe open seam tube are bonded by high frequency induction welding,wherein the strip edges are heated by a high frequency alternatingcurrent and an improved impeder is located within the tube. The improvedimpeder disclosed herein includes a bracket support extending downwardlybetween the opposed edges of the open seam tube and a body portion orcasing which extends axially within the tube, opposite the inductioncoil. The paint applicator wand preferably extends through the axis ofthe body portion of the impeder, inside an annular outer chamber whichincludes the ferrite rod or rods. The preferred embodiment of theimproved impeder is liquid cooled and includes an outer annular chamberwhich contains the ferrite rods and the circulating liquid coolant. Thepaint applicator rod then extends through the impeder and downstream inthe tube to a nozzle, which preferably projects downwardly to spray aprotective coating or paint over the seam. In the most preferredembodiment of the impeder, the paint tube extends through the axis ofthe impeder body portion and a gas annular passage surrounds the painttube, inside of the outer ferrite annulus. An inert gas, such asnitrogen, is circulated through the inner annulus to flood the weld,which also further protects the paint tube from overheating.

The method of this invention may further include coating the exterior ofthe tube with metal, such as zinc, by immersing the moving tube inmolten metal. As will now be understood, the location of the seam in thelower portion of the tube thus assures better coating of the seam whenthe tube emerges from the molten metal bath. That is, the molten metalflows downwardly over the seam, fully coating the seam.

The improved galvanizing bath of this invention provides a substantiallyinert atmosphere for the coating of the external surface of the tube andis specifically designed for continuous or in-line operation. For easeof description, the term "galvanizing" is used to describe coating thetube with zinc or other molten metal including aluminum. The term "inertatmosphere" is used herein to describe an atmosphere which is inert tothe process, which may be created by reducing air by substantiallyeliminating oxygen or introducing an inert gas, such as nitrogen,sometimes referred to herein as a "non-oxidizing atmosphere" and the gasas a "non-oxidizing gas". The galvanizing bath includes a sealed housingassembly having a lower reservoir of molten zinc and an upper chamber. Asealed galvanizing tank is located within the sealed housing assembly.Molten zinc is pumped from the reservoir of the housing assembly intothe galvanizing tank and the housing and galvanizing tank includecoaxially aligned inlets and outlets which receive the tube to be coatedbeneath the level of the molten zinc in the galvanizing tank. Inert gas,preferably nitrogen, is introduced under pressure separately into thesealed housing assembly and the separately sealed galvanizing tank. Therelative pressures are then controlled, such that the inert gas pressurein the housing assembly is less than the pressure in the galvanizingtank and preferably grater than the housing. This improvement not onlyassures a substantially inert atmosphere in the galvanizing tank, butalso permits limited access to the sealed housing assembly for service,including replenishing the zinc and maintenance.

The preferred embodiment of the galvanizing bath apparatus of thisinvention includes a separate sealed preheat chamber wherein the tube isheated to near the temperature of the molten zinc. An inert gas isintroduced under pressure into the preheat chamber, preferably tomaintain a pressure which is less than the pressure of the inert gas inthe galvanizing tank. Inert gas leakage through the inlet and outlet ofthe galvanizing tank and the housing assembly is preferably reduced bythe improved wiper nozzle assembly of this invention. In the disclosedtube forming and coating process of this invention, annular wipernozzles are provided to reduce inert entry of an oxidizing gas and whichmay also be used to control the configuration of the zinc coating on theexternal surface of the tube.

The improved wiper nozzle of this invention directs gas under pressureaxially over an outer surface of the tube, which moves axially throughthe wiper nozzle. The wiper nozzle includes an axial bore which closelyreceives the tube and an annular gas chamber which extends axiallyaround the bore. The annular gas chamber includes an inlet chamberspaced from the bore having a gas inlet and an outlet chamber whichtapers axially and radially inwardly toward the bore which includes anannular outlet surrounding the tube having a radial width substantiallyless than the radial width of the inlet chamber directing gas underpressure axially and radially inwardly over the tube. In the continuoustube forming and galvanizing process of this invention, an inert gas,preferably nitrogen, is received under pressure in the inlet chamber andan annular jet of gas is directed radially inwardly and axially over thetube, preferably counter to the direction of movement of the tube. Wherethe wiper nozzle is located at the inlet to the galvanizing tank, forexample, the gas reduces the entry of an oxidizing gas, such as oxygenwhich might otherwise be carried with the tube into the galvanizingtank.

The wiper nozzle of this invention may also be utilized to shape themolten zinc on the exterior of the tube. For example, the annular exitor outlet of the wiper nozzle may be oriented eccentrically, such thatthe pressure on the lower portion of the tube is greater than thepressure on the upper portion of the tube, such that a greaterproportion of the molten zinc on the tube is wiped away in the lowerportion of the tube, than the upper portion of the tube. As the tubecontinues out of the galvanizing apparatus, gravity will even out thethickness of the zinc coating at the top and bottom of the tube,providing a more uniform coating for the tube. Where the shaping nozzleis located at the outlet of the galvanizing tank enclosure, an inert gasis used. Alternatively, the shaping nozzle may be located outside thegalvanizing tank in a separate enclosure, in which case air is used.Zinc coatings on in-line galvanizing tube mills are now generallygreater in the lower portion of the tube than the upper portion.

Other advantages and meritorious features of the continuous tube formingand coating process and apparatus of this invention will be more fullyunderstood from the following description of the preferred embodiments,the claims and the drawings, a brief description of which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram illustrating a preferred embodimentof the continuous tube forming and coating process of this invention;

FIG. 2 is a side cross-sectional view of a second embodiment of a tubewelding and seam painting station;

FIG. 3 is a side cross-sectional view of FIG. 2, in the direction ofview arrows 3--3;

FIG. 4 is an end cross-sectional view of FIG. 2, in the direction ofview arrows 4--4;

FIG. 5 is an end cross-sectional view of FIG. 2, in the direction ofview arrows 5--5;

FIG. 6 is an end view of the impeder shown in FIG. 2 in the direction ofview arrows 6--6;

FIG. 7 is a side partially cross-sectioned and schematic view of afurther embodiment of an improved galvanizing bath apparatus;

FIG. 8 is a cross-sectional view of an improved gas shaper seal;

FIG. 9 is an end cross-sectional view of the gas seal shown in FIG. 8,in the direction of view arrows 9--9;

FIG. 10 is a side partially cross-sectioned view of an alternativeembodiment of the improved impeder of this invention;

FIG. 11 is an end cross-sectional view of the impeder shown in FIG. 10,in the direction of view arrows 11--11;

FIG. 12 is a cross-sectional view of the impeder mounting bracket shownin FIG. 10, in the direction of view arrows 12--12;

FIG. 13 is an end view of the impeder shown in FIG. 10;

FIG. 14 is a side partially cross-sectioned view of an alternativeembodiment of the impeder assembly shown in FIG. 10;

FIG. 15 is a side cross-sectional view of a gas seal;

FIG. 16 is an end partial cross-sectioned view of the gas seal shown inFIG. 15, in the direction of view arrow 15--15; and

FIG. 17 is an end cross-sectional view of the busing seal shown in FIG.14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE TUBE FORMING AND COATINGPROCESS AND APPARATUS OF THIS INVENTION

Referring now to the partially schematic flow diagram of FIG. 1, it willbe noted that the tube forming and coating process and apparatus of thisinvention are particularly, but not exclusively adapted for processingof endless lengths of untreated strip steel, such is normally processedby a continuous tube forming mill. The improvements described herein canalso, however, be used with precoated or galvanized strip and certainimprovements may also be used in a batch or noncontinuous process.

Metal strip, preferably strip steel, 20 is supplied to the tube formingmill in coils, which are mounted on a pay-out reel 24. The coil 22 ismounted for free rotation on the reel 24 as required by a continuoustube forming mill. As will be understood by those skilled in the art,the strip steel is processed by the mill substantially continuously at aconstant rate. The advancement of the flat strip or skelp 20 through themill is effected primarily by engagement between the strip and theforming and sizing rolls which rotate at a relatively constant speed.The strip is thus drawn into the mill from the pay-out reel 24.

Because the length of each coil of strip steel is taken up by the millin a relatively short time, a means must be provided for splicing theend of one coil to the next, which is accomplished at the splicingstation 26. In the splicing station 26, the end of the reel beingprocessed in the mill is sheared to provide a square end and welded tothe end of the next reel. An accumulator 28 in the form of a loop isprovided having sufficient length to continuously feed the strip to themill, while the trailing end is held stationary for shearing andwelding. The loop may be formed by feeding the strip over a series ofrollers (not shown), which are mounted to freely rotate as is well knownin the art. When the splice is complete, the strip is again paid-outover the accumulator rolls for the next splicing operation.

Because the strip, as received, normally includes oil and may includeother contaminates, it is necessary to clean and prepare the strip priorto painting, welding and galvanizing, which in the disclosed process isaccomplished at cleaning station 30. In a typical application, the stripis cleaned and prepared by a combination of alkaline and phosphatewashes, with intermediate water rinses. In the present process, thestrip is also dried. The cleaned strip is now ready for painting.

In the tube forming and coating process of this invention, at least theinside of the tube is coated with a protective coating or paint prior towelding the seam. However, as will be understood by those skilled in theart, the strip may be precoated as received on the reel 22. Theprotective coating or paint may be applied to one surface of the stripby any one of several known methods, including, for example, a paintcurtain coating apparatus, a roller or the like. It should be noted,however, that only the flat surface is coated because the lateral edgesof the strip should be free of paint. If the lateral edges of the stripare coated, the coating may interfere with the welding of the tube andshould be removed in an edge treatment station. Following coating of thestrip at station 32, the painted surface is dried usually by heating,which may be accomplished by the radiant heater 36 or other suitablemeans, including ultraviolet light. Further, a drying loop 34 isprovided, such that the coating is fully dried prior to the next step inthe process, which is edge conditioning. As will be understood, thepreferred coating or paint will be depend upon the ultimate use for theseamed tube. A suitable ultraviolet curable paint or coating havingimproved slip quality or reduced friction for electrical conduits isavailable from PPG. Where the tube is used as a conduit for electricalwiring or the like, a conventional water based paint may be preferred.An ultraviolet lamp 36 may be preferred for drying ultraviolet sensitivepaints.

In a conventional mill, the strip 20 preferably has a width which isslightly greater than required to form the tube, such that an edge isavailable on each side for proper sizing of the strip and to providefreshly cut metal at the abutting edges forming the seam. A conventionaledge shaver may be used in the edge conditioning station 38 which may beused to cut a square edge. More preferably, a chamfered edge is providedto provide relief for the forged upset and assure good welding contactbetween the edges, as described below.

The flat metal strip or skelp is then rolled into a tube-shaped strip oropen seam tube 42 having adjacent or nearly abutting lateral edges atthe forming station 40. The metal strip 20 is progressively formed as itpasses between rolls 44 which are rotatably supported on vertical andhorizontal axles (not shown) in a conventional manner. However, in thecontinuous tube forming and coating process of this invention, thelateral edges of the strip are bent downwardly and inwardly toward onean other as the tube is formed, rather than upwardly as in aconventional tube forming mill. The lateral edges of the strip are thenrolled into nearly abutting relation in the lower portion of the openseam tube 42, but the adjacent lateral edges 46 are slightly spaced, asbest shown in FIGS. 4 and 5. The open seam tube 42 is then received inthe weld and seam paint station 50, as now described.

The preferred embodiment of the tube welding apparatus of this inventionutilizes high frequency induction to heat the opposed edges 46 of thetube. As shown in FIG. 1, the induction welding apparatus includes awork coil 52 which is connected to a source of high frequencyalternating current. The work coil creates a strong magnetic field,which in turn induces current in the tube, adjacent the work coil. Animpeder 54 is located within the open seam tube 42. A conventionalinduction welding impeder consists of a nonmetallic tube surrounding oneor more ferrite rods. Water or mill coolant is circulated over and pastthe ferrite rod or rods to remove the heat produced by magnetichysteresis and eddy current losses. At the frequencies used forinduction welding (200-800 kHz), current flows around the tube and alongthe "Vee" formed by the approaching edges of the strip, heating theedges to a hot forging temperature, whereby the edges are at leastpartially melted. The edges are then forged together by the squeezerolls 58 to form an integral seam as shown in FIG. 3. Where the strip issteel, the temperature of the edges will be about 2300° F., or greater.A conventional scarfing tool 60 then removes the flash 62 from the outerportion of the seam, and shown in FIG. 1. The impeder 54 includes asupport or bracket portion 56 which extends downwardly between theopposed adjacent edges 46 of the open seam tube 42 and supports theimpeder body portion 54 within the tube. In the preferred embodiment ofthe impeder, an inert gas, preferably nitrogen gas is injected underpressure through a port in the bracket 56 which communicates with anaxial passage in the body portion 54 of the impeder, as shown by thedotted line in FIG. 1.

A source of nitrogen gas under pressure 64 is connected by line 66 tothe impeder support or bracket 56. The free end of the impeder thenincludes an opening which floods the weld zone adjacent the squeezerollers 58 with nitrogen gas. The preferred embodiment of the weld andseam paint station 50 further includes a coating applicator or paintwand which applies a protective coating to the inner surface of theseam. As described above, where the strip has been painted as shown at32 in FIG. 1, the paint will burn off the weld zone because of theextreme temperatures induced in the nearly abutting edges of the openseam tube. The paint wand 70 extends upwardly between the opposed edgesof the open seam tube, then axially over the impeder 54 to a coatingapplication nozzle 72 which preferably projects downwardly to apply acoating or paint under pressure over the seam which is located in thelower portion of the tube. The paint wand 70 is connected to a source ofpaint under pressure 74.

Because the seam 64 is located in the lower portion of the tube, liquidcoating material will flow downwardly over the seam to accumulate in thelower portion of the tube and saturate the roughened seam, forming agood adherent coating. This coating is superior to the coating achievedwhere the seam is located at the upper portion of the tube becausesurface tension draws the liquid paint downwardly away from the seam.Further, the nitrogen introduced over the weld reduces oxidation of theweld, producing a better surface for coating by the paint applicator.Finally, the preferred embodiment of the weld and paint applicator 50 ofFIG. 1 includes a blower 76 having a nozzle 78 which blows air, or aninert gas into the tube, opposite to the direction of travel of thetube. The blower 76 is connected to a source of gas under pressure 80,such as a compressor. The blower dries the inner surface of the tube,prior to welding and blows any small debris through the space betweenthe opposed edges of the tube downwardly, improving the general qualityof the weld.

FIG. 2 illustrates a further improved embodiment of the impeder 154,wherein the paint applicator tube 158 extends axially through theimpeder, as shown in FIG. 4. The trailing end 160 of the applicator tubeextends out of the free end of the impeder 154. The paint tube 160 maybe supported on skates 162. The free end of the tube includes a nozzle164 which extends downwardly to spray the lower inner surface 166 of theseamed tube 82. The skates may be formed of any friction resistantmaterial, such as nylon. The impeder 154 includes a support or bracketportion 156 which extends between the edges 46 of the open seam tube asshown in FIG. 4 and described above.

FIGS. 10 to 13 disclose a further preferred embodiment of the improvedimpeder assembly 254 of this invention. The improved impeder 254includes an outer casing 256 formed of a nonmetallic, nonconductivemagnetically permeable material. Inside the outer shell 256 is a firstouter annular chamber 258 which contains ferrite rods 260, a secondannular chamber 262 located inside the outer annular chamber 258 whichreceives an inert gas, and an innermost tube 264, which receives liquidcoating material. As will be understood by those skilled in the art, theferrite rods 260 are preferably located in close proximity to the casing256. In the disclosed embodiment of the impeder, the ferrite rods 260are arranged in a ring and radially spaced adjacent the shell 256. Theferrite rods are supported in a cradle 268 formed of a nonconductive,nonmetallic material. As shown in FIG. 12, the cradle which may beformed of a heat resistant thermoset plastic further divides the outerannular chamber 258 into an outer portion and an inner portion. Liquidcoolant, which may be water or a light oil, is circulated around theferrite rods and cradle by injecting the coolant under pressure into oneof the ports 270 or 272 and out through the other port. An inert gas,preferably nitrogen is injected into the inner ports 274 into the secondchamber 262, which is defined in the disclosed embodiment by tube 276,as shown in FIGS. 10 and 11. The inert gas tube 276 has an exit oroutlet 278 at the free end of the impeder 254, as shown in FIG. 10. Asshown in FIGS. 1 and 2, the free end of the impeder (54 in FIG. 1 and154 in FIG. 2) is located adjacent the squeeze rolls 58, where thelateral edges of the tube-shaped strip are forged and welded together.The location of the weld in the bottom of the tube reduces the flash,particularly in thicker weld tubes, because the molten metal is notdrawn downwardly. The introduction of a nonoxidizing or reducing gas atthis location reduces the oxidation of the molten metal flash at theinternal surface of the welded seam.

In the disclosed embodiment, the central axial paint tube 264 issupported on radial fins 280 preferably formed of a heat insulatingmaterial. As described above, the liquid paint or other liquid coatingmaterial is transmitted through a central axial tube 264 from a sourceof liquid coating material 282. In the disclosed embodiment, the liquidcoating material is pumped through line 284 to axial tube 264, where itis transmitted to the nozzle assembly 286 at the free end of the tube264. As will be understood by those skilled in the art, the tube weldinglocation is a very hostile environment to a coating material, such as awater base paint. Thus, it is necessary to insulate the liquid paintfrom the hot ferrite rods 260 and the weld zone. The most preferredembodiment of the impeder shown in FIG. 10 insulates the paint tube 264in a jacket of flowing nitrogen gas, which is contained in the secondannular chamber 262 and the outer coolant chamber 258, which containscirculating liquid coolant. Thus, the paint tube is insulated from thehot ferrite rods and the weld zone, avoiding plugging of the paint tube.Further, the tube 264 extends downstream to a point where the weld hascooled sufficiently to accept the paint without burn-off.

FIGS. 14 and 17 illustrate an alternative embodiment for the free end ofthe impeder assembly 354. Except as now described, the impeder 354 maybe identical to the impeder 254 described above and similar elements arenumbered in the same sequence. As shown, the inert gas tube 376 extendsthrough the free end of the outer casing 356 and includes a first outlet390 adjacent the free end of the impeder casing 356 which creates aninert atmosphere over the weld, reducing oxidation as described above.In this embodiment, however, the gas tube 376 extends to adjacent thenozzle or outlet 386 of the paint tube 364. Inert gas is thus floodedinto the paint zone, displacing oxygen and reducing the likelihood ofignition of paint fumes. Further, the gas tube 376 provides a protectiveshroud over the paint tube to adjacent the outlet which, as describedabove, is located far downstream of the weld zone. Thus, the paint tubeis substantially fully insulated. In most preferred embodiment, the freeends of the paint applicator tube 364 and the gas tube 376 are supportedin the seamed tube 82 by a generally cylindrical bushing seal 385. Asbest shown in FIG. 17, the bushing seal 385 includes a plurality ofradial runners 392 which engage the inner surface of the seamed tube 82,reducing friction. The gas tube 376 preferably extends through thebushing seal 385 as shown in FIG. 14 to provide a vapor thermal barrierbetween the paint zone, wherein the coating is applied by nozzle 386,and the weld zone which includes the impeder 354. The bushing seal 385also reduces the draw-out of nitrogen from the tube. Paint tube 364 maybe supported on radial support fins 390, as shown in FIG. 17. Thebushing seal 385 is preferably formed of a nonmetallic reduced frictioninsulating thermoset plastic, such as a filled nylon and the radialsupport fins 390 may be formed on a nonconducting insulating fibrousmaterial.

As described above, the support or bracket portion 295 extendsdownwardly from the body portion of the impeder 254, as shown in FIGS.10 to 13, between the nearly abutting edges of the open seam tube 42, asshown in FIG. 1. The impeder shell 256 then extends downstream to theweld zone to induce a current in the tube, as described above. Theimproved impeder of this invention, however, provides multiplefunctions, including protecting the paint tube and supplying an inertgas to the weld zone and paint area, if required.

Following welding, the welded tube 82 enters a cleaning station 84,wherein the external surface of the weld is cleaned following scarfing.In a typical application, the welded seam is first cleaned and roughenedwith a rotating wire brush to remove scale, then cleaned with muriaticacid, which further removes any oxides, then thoroughly rinsed. The tubeis now ready for external galvanizing, which occurs at the galvanizingstation 86.

Prior to galvanizing, the tube is heated to a temperature approximatingthe temperature of the molten zinc in the galvanizing tank, or about850° F. in the heat enclosure 88. Although any suitable means may beused to heat the tube, including, for example, conventional internal andexternal heaters, the preferred method utilizes an induction heaterhaving coils 90, which induces a current in the tube, as describedabove. An inert gas, preferably nitrogen, is injected into the heatedenclosure 88 through line 92 from a source of nitrogen gas underpressure 94. The tube 82 enters the heat chamber 88 through a nitrogengas seal 96, which will be described hereinbelow. The preheated tubethen enters sealed housing 98 through coupling 100. The housing 98includes a lower reservoir 102 which is filled with molten zinc.Nitrogen gas is injected into the upper chamber of the housing throughline 106. The tube then enters the galvanizing tank 108 through aconventional gasket 110. Molten zinc is pumped from the lower reservoir102 of the housing into the galvanizing tank 108 by pump 112. The tube82 thus enters the galvanizing tank 108 below the level of the moltenzinc as shown in FIG. 1; however, nitrogen gas is injected into theupper portion of the galvanizing tank through line 106, maintaining aninert atmosphere in the galvanizing tank to reduce oxidation and dross.The tube 82 then emerges from the galvanizing tank through a nitrogenseal 116 into the upper chamber 104 of the housing and the tube is thenreceived through a final nitrogen seal 116, which is preferably ashaping seal.

As described above, others have recognized the advantages of maintainingthe galvanizing tank or pot in an inert atmosphere. However, suchefforts have not been fully successful because the housing 98 must beperiodically entered to replenish the zinc and for maintenance. In thegalvanizing bath apparatus of this invention, however, the galvanizingtank is sealed and located within a sealed housing. Thus, the housing 98may be entered to replenish the zinc, for example, without exposing thegalvanizing tank to an oxidizing atmosphere, which would seriouslydamage the quality of the zinc coating on the tube. Further, it ispossible to control the pressure of the inert gas in the heater 88, thehousing 98 and the galvanizing tank 108. In the most preferredembodiment, the inert gas pressure in the galvanizing tank 108 isgreater than the gas pressure in the housing 98, such that air or othercontaminating gas will not leak back into the galvanizing tank.Similarly, the pressure in the heat chamber is greater than the pressurein the housing 98 to avoid leak back of oxidizing gas into the heatchamber.

FIGS. 15 and 16 illustrate a nitrogen seal 96, which may also beidentical to the nitrogen seal 116. Nitrogen shaping seal 116 may beidentical to the seal 96 or more preferably configured to shape the zinccoating on the tube as described in regard to FIGS. 8 and 9.

The nitrogen seal includes an inner housing member 118 and an outerhousing member 120 which is axially threaded on the inner housing member188 by the cylindrical threaded portions 122. The nitrogen seal includesan annular gas chamber 124 and a cylindrical bore 126 which closelyreceives the seamed tube 82 which is moving in direction of arrow 127.The annular gas chamber includes a generally cylindrical inlet portion128 having a radial gas inlet 130 and a conical outlet portion 132,which is defined by converging cylindrical walls which taper toward anannular restricted exit or outlet 134.

An inert gas, preferably nitrogen, is received under pressure into theinlet portion 128 of the gas chamber through port 130. The gas is thenaccelerated through the outlet portion 132 where it is directed underpressure radially inwardly and axially over the outer surface of thetube 82, creating an inert gas barrier preventing entry of an oxidizinggas, such as the oxygen in air, from entering the heat chamber 88. Thus,the gas is directed countercurrent to the movement of the tube in theheat chamber. A similar nitrogen gas seal may be used for the seal 116.Further, the seal 116 may preferably be a nitrogen shaping seal, asdescribed below in regard to FIGS. 8 and 9.

The remainder of the process disclosed in figure may be conventional.Following galvanizing, the tube enters a liquid cooling bath 136 whereit is immersed in a coolant, such as water, at a temperaturesufficiently low to substantially complete the solidification of thezinc and render the coated tubing susceptible to ready processingwithout damage to the galvanized surface. Following zinc solidification,the tube enters sizing and straightening rolls at 138, where the finalexternal configuration of the tube is formed. Thereafter, the tube isflooded with an aqueous chromate solution and rinsed, further coolingand cleaning the tube. In a conventional tube mill, the tube is thenmarked at 142 with product identification and further markings as may bespecified by the customer. A clear lacquer coat or other protectivecoating may then be applied at the OD paint station 144 and the tube isfinally cut to length at station 146.

FIG. 7 illustrates an alternative embodiment of a galvanizing systemwhich includes a gas shaping seal as shown in FIGS. 8 and 9. The processis, however, very similar to the galvanizing process shown in FIG. 1.The seamed steel tube 82 first enters a heat chamber 200 having a heaterinduction coil 202 therein, which heats the tube to the temperature ofthe molten zinc in the galvanizing tank 204. The heat chamber includes anitrogen gas seal 206 which directs nitrogen gas under pressurecounter-currently to the flow of the tube which is shown by arrow 208.An inert gas, preferably nitrogen, is supplied to the nitrogen seal 206by line 208 from a source of nitrogen 210 under pressure. Nitrogen gasis also supplied under pressure through line 212 to heat chamber 200.The tube then enters the sealed galvanizing housing 214 through coupling216. As described above, the galvanizing housing includes a lower moltenzinc reservoir 218 and an upper chamber 220. Nitrogen gas is injectedinto the upper chamber 220 through line 222 from source 210. Aconventional heat resistant pump 224 pumps molten zinc from reservoir218 into the lower portion of the galvanizing tank or pot 204. Nitrogengas is injected into the upper portion of the tank 204 through line 226from source 210. The galvanizing tank further includes a plurality ofoverflow vents which permit the zinc to return to the reservoir 218 in aconventional manner.

As described above, the inert gas pressure in the galvanizing tank 204is preferably greater than the inert gas pressure in the upper portionof the housing 220 and the heat chamber 200, such that an oxidizing gasdoes not reach the tube in the galvanizing tank. It is conventional toprovide a cover 230 on the housing 214 to permit access to the housingto replenish the zinc, for example, in the reservoir 218 and forservice. In such applications, however, opening the cover 230 results inflooding of the zinc pot with air and therefore oxygen. This results inpoorly galvanized tube and therefore substantial waste product. Where,however, the galvanizing tank 204 and the heat chamber 200 arecontiguous and separately sealed within the sealed housing 214 and thenitrogen gas in the tank is maintained at a pressure greater than thehousing and heat chamber, the zinc coating on the tube is not affectedby limited access to the housing.

FIGS. 8 and 9 illustrate a gas shaping seal 232 which is located at theoutlet of the housing at 116 in FIG. 1 or at 232 in FIG. 7. The nitrogenshaping seal may be similar to the nitrogen seal 96 shown in FIGS. 15and 16, except that the outlet portion 332 of the annular gas chamber328 is eccentric to shape the molten zinc coating, as now described.

As described above in regard to FIGS. 15 and 16, the gas seal includesan inner housing member 318 and a threaded outer housing member 320which is threaded to the inner housing member at threads 322. An annulargas chamber 324 is located between the housing members including agenerally cylindrical inlet chamber 328 and an outlet chamber 332 whichis defined by converging conical walls. In the embodiment of thenitrogen shaping seal 232, however, the outer conical wall 331 iseccentric to the inner conical wall, as best shown in FIG. 9. Further,the gas chamber is divided into a plurality of chambers by radialbaffles 335. The disclosed embodiment includes three radial baffleswhich divide the gas chamber 324 into three generally axial components,each having an inlet port 330 and a gas fitting 337 as shown in FIG. 10.

The seamed metal tube 82 thus enters the gas shaping seal throughcylindrical bore 326 which preferably closely receives the tube toreduce gas leakage therethrough. Nitrogen gas is then received underpressure through gas connectors 337 in each of the radial ports 330 intothe inlet portion 328 of one of each of the annular gas chambers. Theembodiment shown includes three chambers divided by the radial baffles335. The gas in the lower portion of the annular exit 334a is ejected ata substantially greater pressure than the upper portion 334b. Thisvariance in pressure shapes the molten zinc on the tube, such that thezinc coating on the upper portion of the tube is thicker than the lowerportion of the tube. As the tube continues in the line, however, thezinc flows downwardly, redistributing the zinc coating to asubstantially even coating, which is the desired result. As will beunderstood, however, where the zinc coating is initially substantiallyevenly distributed on the tube, gravity will cause the zinc to flowdownwardly over the tube, resulting in a tube having a greater thicknessin the lower portion, than in the upper portion, which is solved by thegas shaping seal of this invention. Alternatively, the gas shaping sealmay be located in a separate chamber immediately following the exit ofthe tube from housing 214. In such embodiment, air is received underpressure in inlet chamber 328 through ports 330 and the molten coatingis shaped by air pressure.

The gas pressure in the outlet portion of the nozzle (132 in FIGS. 15and 16 and 332 in FIGS. 8 and 9) may be adjusted by relatively threadingthe inner and outer portions of the nozzle assembly. It is thereforepossible to adjust the gas pressure exiting the nozzle, as required. Forexample, if the outer housing member 120 is threaded to the right inFIG. 15, the width of the annular exit 134 is reduced, increasing thevelocity of the gas exiting the nozzle. Similarly, if the outer housingportion 320 is threaded to the left in FIG. 8, the semi-annular exits334 will be increased in width, reducing the pressure of the gas exitingthe shaping seal.

As will be understood by those skilled in the art, various modificationsmay be made to the tube forming and coating process and apparatus ofthis invention and the improvements may be used separately, or incombination. For example, the improved impeder 54 having an inert gasoutlet at its free end may be used in a conventional tube forming mill.Further, the seamed tube formed by the process of this invention may beexternally painted, rather than galvanized. The advantage of forming theseam in the lower portion of the tube rather than at the top will beparticularly advantageous in a tube forming mill wherein the externalsurface of the tube is painted because the paint will flow downwardlyover the seam and accumulate in the lower portion of the tube, asdescribed. Similarly, the zinc coating flows downwardly over the tube,coating the seam in the galvanizing application described herein. Theseam does not, however, have to be located at the exact bottom of thetube to achieve the advantages described herein. That is, the seam maybe located in the lower half of the tube. More preferably, the seam islocated in the bottom one-third of the tube for such applications.

We claim:
 1. A continuous method of forming a seamed metal tube from acontinuously moving metal strip, said tube including an exterior surfacecoated with a protective metal coating having a melting temperaturesubstantially lower than the melting temperature of said metal tube,comprising the following steps:continuously rolling and forming saidmetal strip into a tube-shaped strip having spaced adjacent lateraledges; heating, melting and continuously integrally bonding said edgesof said tube-shaped strip, thereby forming a continuous metal tubehaving a continuous longitudinal seam formed by said bonded edges;locating said seam in a lower portion of said tube; continuouslyimmersing said tube in molten coating metal with said seam located insaid lower portion of said tube; removing said tube from said moltencoating metal, said molten coating metal then flowing downwardly oversaid seam thoroughly coating said seam; and cooling said molten coatingmetal below the melting temperature of said molten coating metal,thereby forming a seamed metal tube having an exterior surface coatedwith said coating metal covering said seam.
 2. The method of forming acoated metal tube as defined in claim 1, wherein said method includesflooding said tube-shaped strip with a non-oxidizing gas during heating,melting and continuously integrally bonding said edges of said strip,thereby limiting oxidation of said seam.
 3. The method of forming acoated metal tube as defined in claim 1, wherein said method includesremoving excess metal from said seam and roughening the exterior surfaceof said seam following integrally bonding of said edges of said strip,then immersing said tube in said molten coating metal and removing saidtube from said molten coating metal, said molten coating metal thenflowing downwardly over said roughened seam forming a good bond betweensaid coating metal and said seam.
 4. The method of forming a coatedmetal tube as defined in claim 1, wherein said method includespreheating said tube prior to immersing said tube in said molten coatingmetal.
 5. The method of forming a coated metal tube as defined in claim1, wherein said method includes spraying a coating material on theinterior surface of said seam with said seam located in a lower portionof said tube following integrally bonding said edges of said strip bydirecting a stream of coating material downwardly over said seam.
 6. Themethod of forming a coated metal tube as defined in claim 5, whereinsaid method includes locating said adjacent edges of said tube-shapedstrip in a lower portion of said tube-shaped strip, then heating,melting and integrally bonding said edges, thereby forming a continuousmetal tube having a continuous longitudinal seam in a lower portion ofsaid tube prior to immersing said tube in said molten coating metal. 7.A continuous method of forming a seamed metal tube from a continuouslymoving metal strip, said tube including an exterior surface coated witha protective metal coating having a melting temperature substantiallylower than the melting temperature of said metal tube, comprising thefollowing steps:continuously rolling and forming said metal strip into atube-shaped strip having spaced adjacent lateral edges located in alower portion of said tube-shaped strip; heating, melting andcontinuously integrally bonding said edges of said strip, therebyforming a continuous metal tube having a continuous longitudinal seamformed by said bonded edges in a lower portion of said tube; removingexcess metal from said seam and roughening the exterior surface of saidseam; continuously immersing said tube in molten coating metal with saidseam located in said lower portion of said tube; removing said tube fromsaid molten coating metal and said molten coating metal then flowingdownwardly over said roughened seam thoroughly coating said seam; andcooling said tube below the melting temperature of said molten coatingmetal, thereby freezing said molten coating metal with said moltencoating metal covering said seam and bonded to said seam.
 8. The methodof forming a coated metal tube as defined in claim 7, wherein saidmethod includes flooding at least said adjacent edges of saidtube-shaped strip with a non-oxidizing gas during heating, melting andcontinuously integrally bonding said edges of said strip, therebylimiting oxidation of said seam.
 9. The method of forming a coated metaltube as defined in claim 7, wherein said method includes spraying theinterior surface of said seam with a protective coating materialfollowing integrally bonding said edges of said strip and cooling ofsaid seam below the vaporization temperature of said protective coatingmaterial by directing a stream of said protective coating materialdownwardly over said seam.
 10. A continuous method of forming a seamedmetal tube from a continuously moving metal strip, said tube havinginterior and exterior surfaces coated with a protective coating, saidmethod comprising the following steps:applying an interior coatingmaterial to one surface of said strip; continuously rolling and formingsaid metal strip into a tube-shaped strip having said interior coatingmaterial located on an interior surface thereof and said tube-shapedstrip having spaced adjacent lateral edges; heating, melting andcontinuously integrally bonding said edges of said strip, therebyforming a continuous metal tube having a continuous longitudinal seamformed by said integrally bonded edges and said interior coatingmaterial on an interior surface of said tube; locating said seam in alower portion of said tube; cooling said tube to a temperature lowerthan the vaporization temperature of said interior coating material;applying a coating to the interior surface of said seam by spraying saidcoating material downwardly over said interior surface of said seam;continuously immersing said tube in exterior molten coating metal withsaid seam located in said lower portion of said tube, said exteriormolten coating metal having a melting temperature substantially belowthe melting temperature of said tube; removing said tube from saidexterior molten coating metal, said exterior molten coating metal thenflowing downwardly over the exterior surface of said seam, thoroughlycoating said seam; and cooling said tube below the melting temperatureof said exterior coating metal, thereby freezing said exterior moltencoating metal and forming a tube having interior and exterior surfacecoated with protective coatings including the interior and exteriorsurfaces of said seam.
 11. The method of forming a coated metal tube asdefined in claim 10, wherein said method includes flooding said adjacentedges of said tube-shaped strip with a non-oxidizing gas during heating,melting and continuously integrally bonding said edges of said strip,thereby limiting oxidation of said seam.
 12. The method of forming acoated metal tube as defined in claim 10, wherein said method includesremoving excess metal from said seam and roughening said exteriorsurface of said seam following integrally bonding said edges of saidtube-shaped strip, then immersing said tube in said exterior moltencoating metal and removing said tube from said exterior coating metal,said exterior coating metal then flowing downwardly over said roughenedexterior surface of said seam forming a good bond between said coatingmetal and said seam.